Metamorphic Evolution of the Imjingang Belt, Korea: Implications for Permo-Triassic Collisional Orogeny

摘要

The Imjingang belt in the central Korean Peninsula is a strong candidate to represent the extension of the Dabie-Sulu ultrahigh-pressure (UHP) belt in China. This fold-thrust belt consists primarily of: (1) the northern, Jingok unit, characterized by Barrovian-type metapelites; and (2) the southern, Samgot unit, comprising calc-silicate rocks and amphibolites with or without garnet. The metamorphic grade of the Jingok unit increases southward from garnet through staurolite to kyanite zones. Peak metamorphic conditions estimated from the kyanite zone range up to approx11 kbar and 700°C. In contrast, no significant change in metamorphic assemblages is apparent in the Samgot unit. Characteristic minerals of the calc-silicate rocks include garnet, diopside, hornblende, scapo-lite, and plagioclase, and are compatible with upper amphibolite facies estimated from garnet amphibolite. P-T estimates from the Samgot amphibolites are generally in the range of 8.5-11.5 kbar and 660-780 deg C. These P-T estimates, together with near-isothermal decompression documented from the metapelitic unit, suggest that the Imjingang belt possibly evolved from eclogite-facies conditions along a clockwise P-T path. The timing of peak metamorphism was previously estimated at approx250 Ma, based on Sm-Nd and Rb-Sr mineral isochron ages and monazite chemical ages. This result is corroborated by an ion microprobe (SHRIMP) U-Pb age of metamorphic zircon dated at 253 + - 2 Ma. ~(40)Ar/~(39)Ar ages of hornblende from the amphibolite suggest regional cooling through approx 500 deg C at 230-225 Ma in the Samgot unit. Two biotite separates in the kyanite zone suggest cooling through approx 300 deg C at approx 220 and 160 Ma, respectively, in the Jingok unit. These results, in conjunction with peak metamorphism at approx 250 Ma, indicate that high-P metamorphism in the Imjingang belt occurred during the Permo-Triassic, and that subsequent rapid cooling followed during exhumation of a thickened orogen, producing a clockwise decompression. This P-T-t evolution is compatible with that of the basement gneisses in the Gyeonggi Massif. Thus, we conclude that both the Imjingang belt and Gyeonggi Massif in the Korean Peninsula are products of continental collision at approx 250 Ma and subsequent exhumation at approx 230-220 Ma.